Minerals and mineral-like substances including synthetic substances such as concrete have very low tensile strength. When such substances are suddenly subjected to a reduction in ambient pressure, the substance is placed under tension and tends to fracture along areas of zones of weakness. Such areas or zones include grain boundaries, mineral boundaries, material boundaries, cracks, fractures and zones of high stress. This characteristic of the material has been utilized in vacuum comminuting apparatus such as that described in U.S. Pat. No. 3,255,793 to Clute wherein a fan type blower is supplied with air through a chamber which is of frustoconical configuration so that it increases in cross section from its entrance to the inlet of the blower. During operation of the blower a low pressure zone is created in the conical chamber and the explosive comminution occurs within the conical portion. The patent further states that further comminution may be effected by redirecting the material a number of times through the machine.
U.S. Pat. No. 2,386,401 Joyce, Jr. discloses a method for fiberizing asbestos. The asbestos is first pressurized in a closed chamber and then releasing the pressure suddenly to separate the fibers. U.S. Pat. No. 2,823,868 Scherer discloses a comminuting apparatus wherein the material after mechanical comminution is delivered to a vacuum tank or receiver. This patent refers to the further breaking up of the particles under the reduced pressure in the receiver.
In some processing of minerals and the like wherein pulverization is a step in the process it is required that very fine and uniform particles of the material be obtained. While the processes known heretofore have been suitable for many applications they are not effective for all applications and, accordingly, it is an object of this invention to provide an improved pulverization method and apparatus employing vacuum comminution.
It is another object of this invention to provide an improved vacuum comminution method and apparatus which is simple and effective in operation and in construction.
It is a further object of this invention to provide an improved method and apparatus for vacuum comminution of minerals and the like which is capable of continuous operation and provides an increased degree of pulverization of the mineral at an increased rate of production.
It is a further object of this invention to provide an improved multi-stage method and apparatus therefor, which may be utilized with a single dust collecting apparatus.
The features of novelty which characterize this invention are set forth with particularity in the claims annexed to and forming a part of the specification. The invention itself, however, together with further objects and advantages thereof, will best be understood from the following description taken in connection with the accompanying drawings.